The invention relates to a communication bus system according to the precharacterizing part of claim 1.
The invention also relates to a execution station for use in such a communication bus system.
The IEEE 1394 bus system is a communication bus system (which will be referred to as a 1394 system) which can be applied to connect stations such as TV sets and videorecorders. Chapter 8 of the IEEE 1394 standard describes that an execution station has a number of control status registers (CSR's). The contents of several of those CSR's can be modified in response to request packets from all of the several requesting stations.
To prevent errors, the requesting station which sends a request packet should be able to determine reliably whether the command has actually been executed, especially if the effect of repeated execution of the command differs from that of single execution (e.g. in case of an "increase TV channel number" command or a "reserve capacity" command). When the execution station contains a status register that is affected by execution of the command and that can be read by a command from the requesting station, it is possible, in principle, to determine whether the command has been executed by verifying the contents of the status register. However, verifying the contents of the status register to detect execution does not work reliably when the bus system contains more than one requesting station that can send a request packet with a command that affects the same status register.
In 1394, the execution station after execution of a command returns a response packet to the requesting station which issued the request packet in response to which the command was executed. This allows the requesting station to determine whether the command has been executed, but it does not work when the requesting station fails to receive the response packet.